If you want to buy Terraformed Islands or Summer Race, the required amount is in blue currency. This amount has to be saved in your water park.
If you have enough money to buy a ride but the button is still greyed out, please check the requirements in red next to the BUY button:
Here is the money measure list:
As you see, some of them start with the same letter.
If the game is asking for Quintillions in order to unlock a new upgrade, check your current amount of money, because it it possible you have quadrillions.